Taking control

Controlling exposure

In order to control the exposure and get an accurate image, we need to control the amount of light hitting the film/sensor. Modern cameras measure (meter) the light coming from the scene and automatically calculate the settings to get a correct exposure – most of the time! Understanding how this is achieved means that we can start to take control of the process and both take over when the camera gets it wrong and take more creative control of our images.

How do we control the light entering the camera?

Imagine a bathtub being filled with water from a giant tank overhead. There is a tap in the tank where the water flows out and fills the bath. Let’s say that for the perfect bath we want it half full. In order to fill the bath to just the right level we can either run the tank for a fixed length of time and adjust the opening of the tap or we can leave the tap at the same setting and allow the tank to run for a longer or shorter time.

We know that if we set the tap and go and answer the phone, more time passes than we imagine and the bath overflows.

The same is true for your film/sensor; you can increase the light hitting the film/sensor by allowing it to enter the camera for a longer time or you can keep the time the same but create a bigger hole for the light to pass though (like opening the tap). The converse is true of course if you want less light, a shorter time or a smaller hole.

The overflowing bath is equivalent to an overexposed picture and a bath with too little water is like an under exposed picture.

How do we allow the light to enter the camera for longer or shorter time?

This is a relatively easy concept and is the shutter speed. When you are ready to take your picture, you press the shutter release button; this causes a shutter that is preventing light from entering the camera to open and let light onto the film/sensor. The click you hear when taking the picture is the shutter opening and closing again. Depending on the amount of light from the subject, a shutter could be open from as long as a few seconds to a fraction of a second (for example 1/4000th  second). Typically the range for a normally lit scene would be between 1/60th second and 1/500th second. It is simple to calculate that for every doubling of the shutter speed, the amount of light entering the camera halves. For example 1/250th second is half as long as 1/125th second and therefore half as much light. If you look at the file properties of a digital image it will normally tell you the shutter speed at which it was taken.

How do we control the tap – the hole that allows light through?

The hole that allows the light to enter the camera is called the aperture and we can change the size of this aperture. An aperture that is twice as big will allow twice as much light through. Unfortunately the concept of aperture settings is less simple as it is set in historic nomenclature. The size of the aperture is called the f/stop; confusingly, a bigger f/stop number means a smaller aperture.

There are standard f/stop settings and these would typically be f/4 (a wide aperture), f/5.6, f/8, f/11, f/16, f/22 (very small aperture); each of these steps is half as much light as the previous one. There are more f/stop numbers at either end of the range, usually on more expensive lenses. (You might see other f/stop numbers between these shown above and these are for fine-tuning the aperture but the numbers given above are whole stop changes representing a doubling or halving of the light from the adjacent setting). Again if you look at the file properties of a digital photograph it will show the aperture used as an f/stop number.

Why not just let the camera choose?

If you take a picture on fully automatic, the camera will take a shot that averages most of the settings so, in average lighting conditions it will choose a mid-range shutter speed and mid-range aperture. This is probably fine in most circumstances but when the light is particularly dark or bright or the subject is moving then these might not be the best settings.

For example, with a moving subject, if the camera selects a relatively slow shutter speed such as 1/60th second then the subject will be blurred in the photograph because it has moved during the relatively long time the shutter was opened. This also happens when it is dark; the camera needs to open the shutter for longer to let more light in and any moving subjects can become blurred – I’m sure you’ve seen this if you’ve taken pictures of people dancing in a dark room. This is called motion blur.

The picture below (left) shows drummers, notice how the drummers that are moving quickly are blurred. In this case the camera was on automatic and selected a shutter speed of 1/25th second. In order to freeze the motion of the drummers a shutter speed of 1/800th second was required (right)

Shutter speed = 1/25 sec

Shutter speed = 1/800 sec

 

 

 

 

 

 

 

 

 

 

 

 

Shutter speed = 1/25 sec

At very low shutter speeds (usually 1/60th second or below), even if the subject is not moving, if you are holding your camera in your hand (rather than on a tripod or resting on something) then your hand is not steady enough and the camera moves, causing similar blurring. This is often called camera shake. The picture (left) was taken at 1/25th second, too slow for hand-holding the camera.

 

 

 

 

 

 

 

Of course you might want to get creative and purposely blur part of the image. This is often used to give an impression of speed or movement. The pictures below show a train leaving the station; the left image was taken at 1/100th second; as the train was moving relatively slowly this froze the motion, however the shot on the right was taken immediately afterwards and the shutter was open for 4 seconds. The blurring of the train gives the impression that it was speeding through the station! (I cheated a bit with this one and copied the man on the platform in from the first shot to the second as he was also blurred during the 4 second shot).

 

Shutter speed = 1/100 sec

Shutter speed = 1/100 sec

Shutter speed = 4 seconds

Shutter speed = 4 seconds

 

 

 

 

 

 

 
A very common creative use of shutter speed is to control how flowing water looks:

The below image (left) was taken at 1/25th second and the below image (right) was taken at 1/500th second. Same subject but a very different feel to the final images.

Shutter speed = 1/25 sec

Shutter speed = 1/25 sec

Shutter speed = 1/500 sec

Shutter speed = 1/500 sec

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Just as altering the shutter speed can dramatically affect the look of the final image, so too can altering the aperture. There is an optical phenomenon called depth of field; this describes the amount of the picture that is in sharp focus and the amount that is out of focus. You may have seen pictures with a really blurred and de-focussed background – this is referred to as shallow depth of field, there is an example below.

 

Large aperture (f/5.6)

Large aperture (f/5.6)

 

Here the depth of field is so shallow that only the front petals are in sharp focus and the background is completely de-focussed. Even the stem of the flower is out of focus. This technique really draws attention to the main subject and removes any distracting or cluttered background. A large aperture gives this type of effect.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Small aperture (f/22)

Small aperture (f/22)

 

Conversely if you are taking a landscape for example, you might want everything to be in focus from the close foreground to the far distance – this is described as a large depth of field. The example right shows this; everything from the rocks at the front to the castle in the distance is in focus. To get this effect you need a very small aperture.

 

 

 

 

 

 

 

 

 

 

See what happens as we change gradually from a small aperture to a large aperture:

Aperture = f/22
Shutter speed = 8 seconds

Aperture = f/16
Shutter speed = 4 seconds

Aperture = f/11
Shutter speed = 2 seconds

Aperture = f/8
Shutter speed = 1 second

Aperture = f/5.6
Shutter speed = 0.5 seconds

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Each change in aperture is one stop more open, hence the shutter speed has to be halved each time to keep the overall exposure the same but notice how the depth of field changes. At f/22 it is in focus to nearly 20cm yet at f/5.6 the clear focus extends to less than 5cm.

 

 

 

 

 

Note also that the depth of field changes from using a wide angle lens (where depth of field is greater for any given aperture) to a telephoto lens (where it is much less).

 

Aperture = f/5.6 Focal length = 24mm

Aperture = f/5.6
Focal length = 24mm (wide angle)

Aperture = f/5.6 Focal length = 105mm

Aperture = f/5.6
Focal length = 105mm (telephoto)

 

 

 

This is why when taking landscape pictures and you are usually using a wide angle lens to get lots of the view, it is possible to get a very large depth of field extending miles into the distance.

 

 

 

 

 

 

 

So from all these examples you can see that it is important if you want to control the look of the final image, not just to rely on the automatic setting of the camera. By taking control of the shutter speed and aperture you can avoid common mistakes and also get the picture that you want.

I’ll explain later how you can start to select for either the shutter speed you want, if you want to freeze or blur the motion, or the aperture if you want to maximise the area in focus or de-focus distracting backgrounds. Remember however that by altering one of either the shutter speed or the aperture, you have to change the other one too to keep the amount of light entering the camera (the exposure) constant and correct.

Exercise:

Remember that increasing the shutter speed by 2x allows half as much light to hit the film/sensor; for example 1/250th second is twice as fast as 1/125th second. Therefore if you change the shutter speed from 1/125th second to 1/250th second you are only allowing half as much light to hit the film/sensor and if you keep the aperture the same, the picture will be under exposed. To compensate for this you need to open the aperture up to allow twice as much light to get through; this will keep the overall exposure of the film/sensor the same.

Remember also that the whole steps of aperture openings are typically f/4 (a wide aperture), f/5.6, f/8, f/11, f/16, f/22 (very small aperture); each of these steps is half as much light as the previous one.

So if you took a correctly exposed picture of a moving subject with a shutter speed of 1/125th second and an aperture of f/8 and the movement was blurred you might want to use a faster shutter speed of 1/250th second to try and freeze the motion. If you changed the shutter speed to 1/250th second what aperture setting would you need to set in order to keep the overall exposure the same?

Answer: You would use f/5.6 – you would need to allow twice as much light through the aperture to compensate for the fact that the shutter was only open for half as long. f/5.6 is one whole f/stop wider than f/8.

Taking Control

Mode dial

 

There are a number of ways to take control of what the camera does. The first and easiest is to use are the preset scene modes that are available in the settings. You will see a number of preset modes with symbols to indicate what they are for. The most common ones are:

 

 

 

 

Mode dial-2

 

Sport mode: This tells the camera that you are taking a moving subject and want to freeze the motion (not just sports!). The camera will try to set as fast a shutter speed as is possible in the conditions and select the appropriate aperture to match that shutter speed for a correct exposure. Remember here, because the camera is selecting a fast shutter speed it may need to use a wide aperture to allow enough light onto the film/sensor. Since wide apertures mean shallow depth of field, it is important to make sure you focus carefully on the main subject.

 

Mode dial-3

 

Landscape mode: This tells the camera that you are taking a shot where it is more important to get lots of the image in focus. It will therefore set as small an aperture as is possible in the conditions and select a shutter speed to match for correct exposure. Remember here because the camera is selecting a small aperture it may have to also select a slow shutter speed to allow enough light onto the film/sensor and there is a risk of camera shake – consider using a tripod or steadying the camera.

 

Mode dial-4

 

Portrait mode: In portrait mode, the camera selects a large aperture to create a nice blurry background and draw attention to the main subject. Remember, due to the shallow depth of field it is important to focus carefully; with portraits the eyes are the most important part to get in focus.

 

 

 

Mode dial-5

 

Macro mode: This is designed for close up photography and allows the camera to focus as close as possible. It usually uses a wide aperture to get a shallow depth of field and a nice blurry background. Remember that in this case it is very important to make sure you focus correctly on the main part of the subject.

 

 

Mode dial-6

 

Night mode: Here the camera will select a slow shutter speed due to the low light available. It normally also fires the flash to illuminate subjects close to the camera. Again it is important to keep the camera as steady as possible to prevent movement during the long exposure time and also there is a risk of moving subjects becoming blurred. Occasionally you will see a moving subject being both blurred and sharp; this is because the movement is captured during the long exposure but frozen when the flash fires as this is a very short burst of light.

 

 

 

This can be used creatively:

Night mode Slow shutter plus flash

Night mode
Slow shutter plus flash

 

 

Here a slow shutter speed (0.5 seconds) has been used due to the low light level and this blurred the motion of the dancer but the flash has also fired, freezing part of the motion.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Your camera may have many other scene modes such as snow, sunset, indoors, fireworks etc.

Taking more control

Mode dial-7

 

When you have got to grips with the concepts of aperture and shutter speed and seen what effect each has on the final image, you may want to try setting them for yourself rather than just using the scene modes and letting the camera work it out for you. There are 2 simple ways in which you can do this and these are normally selected from the mode dial on the camera.

 

 

 

Aperture Value (also known as Aperture Priority), symbolised by Av on the dial (some cameras may just have an A)

By selecting Av you are telling the camera that you will set the aperture that you want, for example to control the depth of field and the camera will set a shutter speed to match and give a correct exposure. You choose the aperture value by rotating a wheel or using buttons, depending on the camera. Remember to keep an eye on the shutter speed as if it gets too low, you might get camera shake unless you use a tripod or steady the camera (of course moving objects in the picture, such as grass blowing in the wind may still blur).

A large aperture has created shallow depth of field and given a subtle blurring of the background.

Time Value (also known as Shutter Priority), symbolised by Tv on the dial (some cameras may show S for shutter)

By selecting Tv (or S) you are telling the camera that you will set the shutter speed that you want, for example a fast shutter to freeze motion or a slow one to create milky-looking flowing water. The camera will choose an aperture to match and give a correct exposure. Again, you choose the shutter speed by turning a wheel or using buttons. Remember that the aperture selected by the camera will affect the depth of field and you might need to focus carefully on the main subject.

A slow shutter speed has given a milky appearance to the water.

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Text and images © 2013 David Preston – all rights reserved

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